In its devastating path through Puerto Rico seven months ago, Hurricane Maria may have killed or severely damage between 23 million and 31 million trees, according to research from the US Department of Energy and the University of California at Berkeley.
The study, described by its authors as a post-Maria "rapid assessment," analyzed satellite images combined with processing techniques. For this reason, scientists at the Lawrence Berkeley National Laboratory warned that the impact on the forest cover may be higher.
To this effects, they indicated that field investigations are required, which will be carried out during the year.
The researchers specifically assessed the damage by at changes in the surface reflectance of both visible and invisible light. The human eye can discern colors in the visible spectrum, but when meassuring infrared light, a "much more accurate" image of the impact on the trees is provided, explained Jeffrey Chambers, expert in forest biogeography, who led the team that made the study.
"When the sunlight bounces off green vegetation it looks one way, and when it bounces off vegetation without leaves or fallen trees it is very different. We found dramatic changes in the spectral signature of the forests associated with damage, tree mortality, stripped leaves, and canopies,” said Chambers, scientist in the Earth's Land and Environmental Sciences Laboratory, as well as a professor of Geography at the University of California, Berkeley.
He specified, in written statements, that the researchers
looked at images from Landsat 8, a satellite that takes detailed images of the entire Earth every 16 days, comparing images from before and after the hurricanes and eliminating effects due to clouds and shadows.
They found, among other things, that the damage was not evenly spread in the island's forests.
“The intensity of the spectral shift varied a lot across the island. Now we want to better understand why some forests were more vulnerable than others, and what factors controlled the differences in how forests were impacted. Was it the species, was it the slope, was it the aspect – whether you’re on the windward or lee side as the storm is rotating counterclockwise? The soil type and rooting depth are also important factors.”", Chambers said.
The Laboratory made a similar analysis after Hurricane Katrina in Louisiana and Mississippi. It estimated that 320 million trees died or were severely damaged. Cypresses and tupelos resisted more than oak trees “right next door”, for example.
Ariel Lugo, director of the International Institute of Tropical Forestry of the US Forest Service, is skeptic about the study because it was done "too fast" and did not include field work that precisely would allow to specify how many trees died and how many were damaged.
He also highlighted that in Puerto Rico there are 1,460 million trees and if Maria affected 25 million, it represents 1.7 percent of the total.
"When people saw the forest without leaves, they thought it was dead, but no. It is affected, but not dead. The forests of Puerto Rico are resilient and we see it because they are greening up", he said.
He said that the Forest Service works with three procedures to "estimate exactly" the effects of Hurricane Maria on the forests. The first one is the inventory that the agency makes every five years since 1982, the last edition was completed before María. Due to the hurricane, the inventory was resumed in November and should be completed within a year.
The second procedure is an analysis of satellite images. "We had flown to Puerto Rico before the hurricane with a plane from NASA, with 20 sensors that, from space and at a very high resolution, allow to determine the forest condition. We have just received the funds to repeat the flight after Maria", he said.
Third, the Forest Service sends its technicians to the "study sites" it has on the island since the 1940s. There, the trees are "marked" and their history is known.
"We are going to see specific trees and how they reacted before and after the hurricane," said Lugo.
"This is not a trivial analysis nor can it be done overnight because of the complexity of the forests, the topography and because the effects of the hurricane were heterogeneous", he added.
Meanwhile, Tania Vázquez, Secretary of Natural and Environmental Resources, said that the study presents a "disturbance index", which does not necessarily mean that all the trees fell or died.
"We know that there has been a massive exfoliation in Puerto Rico, but the study was done when the regeneration cycle had not started as we see is happening now," she said, referring to the Landsat 8 images taken between October 1 and 30.
However, Vázquez said that the study gives her “a tool for my investigation of how many trees were affected, that will take more time because the inventory is done manually, and it also helps me with my reforestation project".
She said that reforestation will be "massive" and that will begin in the "next months". The agency lost 90 percent of the trees and seeds in its nurseries, so it is working to establish agreements with other jurisdictions, such as the Dominican Republic, that may provide species for sowing.
For Fernando Lloveras, president of the entity Para la Naturaleza (For Nature), the study served as a reference to create the Habitat program, which aims to sow 750,000 trees in seven years.
"At the scientific methodology level and done by a university, it is the only thing we have right now. But, for me, the result is even a bit conservative compared to what a final study should show", said Lloveras.
He recalled that, in 1920s, it was estimated that the island had lost 96 percent of its forest cover due to agricultural practices. In 2015, forests had recovered by 60 percent.
"We already had the primary responsibility to contribute to the recovery and, after the hurricane, it is even a higher responsibility. We have to repair the damage in order to be prepared for the climate changes that are anticipated", he said.
He specified that Habitat will give priority to native and endemic trees, which will be planted in protected natural areas, forests and urban contexts. The trees will be monitored for five years.
In the same line, Jardany Díaz, geographer, planner and doctoral student of Environmental Sciences, argued that the situation should be used to develop management plans for tree species - native, endemic and introduced -, to used them "according to needs".
"Knowing what species should be planted is beneficial not only for reforestation measures, but also to control erosion and increase biodiversity", he said after highlighting that, with "effective" management plans, introduced species can be part of reforestation.
He stated that management plans should be prepared by the Forest Service, Natural Resources, Para la Naturaleza, the Model Forest Office and communities that co-manage forests.
According to Diaz, the study "also represents an opportunity" to investigate secondary succession in forests and how species regenerate after natural phenomena.
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